Single use syringes are required to reduce the risk of infection caused by needle sharing. Many types of single use syringes are known.
A disadvantage with most of these known syringes is the complexity of manufacture and the rather large number of parts which are required in order to allow the syringe to be used effectively.
This invention is directed to a single use syringe of the type which has a needle which can be shot into a hollow plunger stem upon completion of the plunger stroke to prevent the needle from being use again. In particular, the invention is directed to a relatively simply insert which is fitted into the needle and which can securely hold the needle in its use position but which can release the needle to shoot the needle back into the plunger stem.
These types of syringes are known. One type of syringe which shoots the needle back into the syringe body is extremely complicated and has up to 16 parts which must be assembled to form the finished syringe. Some of the parts need to be glued as the parts are too small for injection or die moulding. This known syringe is also restricted to volumes of larger than 3 ml as the various parts cannot be made to fit smaller syringes.
Another disadvantage with known syringes of this type is the inability to easily and readily injection mould the various components. For instance, a syringe having a shoot back needle is known where a spring biased needle is held in place by a rupturable restraining means which is moulded as part of the internal wall of the syringe barrel. In practice, it is found that such a barrel is extremely difficult to mould and is virtually impossible to injection mould. Thus, this type of syringe cannot be manufactured cheaply.
Another type of safety syringe having a spring mounted needle which shoots back into the plunger stem has a needle held in place by a deformable base. The deformable base has a constricted passageway which grips a peculiarly designed needle which has inter alia a circular flange attached to it. The deformable member is pushed forward by the plunger and in doing so, it is pushed forwardly into an area of larger cross-section which allows the deformable member to expand which in turn allows the needle to pass through the passageway in the deformable member and to shoot back into the plunger stem. It is believed that this type of arrangement would not be particularly reliable as it does not include a more reliable frangible portion which is broken to allow the spring biased needle to shoot back into the plunger stem.
Safety syringes having shoot back needles are also known where the needle is not biased by a spring. Instead, a vacuum is provided in the barrel to suck the needle back into the plunger stem. This type of syringe is of course fairly limited in use to circumstances where suction can be provided.
Another type of syringe has a plunger stem provided with a helical spring such that the plunger stem is biased to retract. The plunger stem is pushed along the barrel and against the bias of the spring. When the front of the plunger stem strikes the rear of the needle, it locks against the needle. Release of the plunger stem will then cause the spring to pull the plunger stem back along the barrel thereby retracting the needle.
It is known to provide single use syringes where the needle is spring biased and is held in place by a cuttable member. In order to ensure that the member is properly cut thereby releasing the needle, a two-part cutting action is usually required. In the two-part cutting action, the plunger typically has a forward cutting edge and the front of the needle also has some form of cutting edge and a double cutting action is required to ensure that the cuttable member is properly cut to release the needle. It is found that attempting to cut a cuttable member to release a spring biased needle is not always reliable and does not always work.
Another type of known single use syringe has a base member which grips an enlargment on the needle. The plunger pushes the base member forwardly which further compresses a spring which is around the needle. The pushing action releases the grip between the base member and the enlargement on the needle. This allows the needle containing the enlargement to be shot back into the plunger while leaving the spring behind. The spring is initially compressed partially but then becomes compressed fully as the plunger forces the base member forwardly. This arrangement requires a needle having a special design (containing an enlargement) which means that conventional needles cannot be used.
Another type of syringe has a plunger seal which moves on the plunger and where the plunger seal slides rearwardly along a bearing surface in response to a force being applied to the seal which is in excess of the operational force of the syringe. This arrangement begins the retraction sequence of the needle. With this arrangement, a specially designed plunger seal is required which moves relative to the plunger. Conventional plungers have a plunger seal which is fixed to the plunger.
Another type of syringe has a shoot back arrangement which consists of two parts which slide relative to each other to release the needle. The two parts consist of an outer part and an inner part. The inner part holds the needle and is biased by a helical spring. The two parts are held together by frictional engagement to each other. A plunger progressively pushes the outer part forwardly thereby progressively reducing the amount of frictional engagement between the outer part and the inner part until such time that the bias of the spring is sufficient to shoot the inner part away from frictional engagement with the outer part. This arrangement requires a sliding frictional grip to hold the two-part together which is considered quite risky and requires careful manufacture.
Another arrangement uses a needle holder having an elongate body portion in front and a head end in back and providing a spring under the head end which circumscribes the needle holder. The use of an elongate body in a needle holder makes this arrangement difficult for use small and confined spaces.